1. Field of the Invention
This invention is relates generally to an injection molding apparatus and, more particularly, to a rod disposed in a manifold melt channel to balance filling of mold cavities.
2. Related Art
The use of manifolds in a hot runner injection molding apparatus is well known. Furthermore, it is well known that in many applications it is important that a runner layout be provided such that each cavity receives the same flow of melt having the same temperature and the same composition. Balancing of the runner system results in overall higher quality molded parts because consistency is achieved from mold cavity to mold cavity in a multi-cavity application. Even in multi-runner, single cavity applications, the benefits of balancing are well known and important.
A well-known technique for balancing a manifold is to match runner diameters and lengths and to match the number of turns in the runners, so that the pressure drop through the manifold to each cavity is the same. However, despite a runner layout having matched runner lengths and turns, unbalanced filling of cavities may occur at least partly due to a combination of shear heating of the melt flow combined with the layout of the runner system.
When melt is forced under pressure through a bore, as is done in a hot runner system, the melt experiences friction or shear in the area adjacent to the channel wall. This results in a localized elevation of the temperature of the melt. The result is a differential in temperature across the bore, with the center of the channel being cooler than the material closer to the bore. Due to this shear difference, the outside of the melt adjacent to the channel wall is less viscous than the center core of the melt. Many hot runner systems split the melt flow from a primary runner through two or more secondary runners. When the melt is split, the heat distribution profile in the melt is divided as well. This occurs because the flow through the runners is laminar, and therefore the shear-heated material remains adjacent to the wall as the corner is turned. After the corner, the heated peripheral portion is no longer annular, but is instead generally crescent-shaped and remains on one side of the melt flow. The mass flow through each of the secondary runners is substantially equal; however, the heated peripheral portion in each secondary runner is asymmetrically distributed about the periphery. If, as is usually the case, each secondary runner is divided into a plurality of tertiary runners, the asymmetric heated peripheral portion may be unequally divided between these plurality of tertiary runners. As a result, the material flowing into one of the tertiary runners from a secondary runner may include a higher proportion of shear-heated material compared to the melt flowing into the other of the tertiary runners downstream from that secondary runner. This phenomenon can, in some applications, cause preferential flow to some drop locations or mold cavities, and can cause poor part quality and out-of-spec product to be produced. Specifically, there will typically be preferential flow to the tertiary runner receiving a higher proportion of shear-heated material from its upstream secondary runner compared to the other of the tertiary runners fed by that secondary runner.
Previous methods of equalizing the flow include mixing or re-orienting the melt.